Molecular beam epitaxy source cell

ABSTRACT

Apparati and methods for varying the flux of a molecular beam emanating from an effusion cell are disclosed. The apparatus includes a means for controllably adjusting the angular distribution of a molecular field effusing from a source material within the effusion cell, therein adjusting the flux of the beam. The method herein disclosed, with respect to the related apparati, including the step of selectively altering the angular distribution of an effusing molecular field, produced by a heated source material, which comprises the molecular beam, thereby varying the flux of the beam.

This is a divisional of application Ser. No. 08/361,961, filed on Dec.22, 1994, now U.S. Pat. No. 5,616,180.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparati and methods for controllingconstituent mass densities within molecular beams, and more particularlyto apparati and methods for rapidly varying the flux rate of epitaxialmaterials from effusion cells during the fabrication of semiconductordevices.

2. Description of Prior Art

Molecular beam epitaxy techniques which permit growing epitaxial layershave been described, and are well known in the art. In a typicalmolecular beam epitaxy (MBE) system, two or more source materials(generally a semiconductor material and at least one dopant material)are separately heated in effusion cells, thereby elevating their vaporpressures, to generate individual beams consisting of molecules (oratoms) of these materials. The individual beams of molecules then travelunder molecular flow conditions toward the surface of a heated substratewhere they react to deposit layers of predetermined composition on thesubstrate surface. The term "molecular flow" is herein used to refer tosuch flow conditions in which individual molecules move withoutundergoing direction altering collisions. Such conditions are mostpractically maintained in an evacuated environment, thus MBE isgenerally carried out in vacuum chambers.

Although widely used, the molecular beam epitaxy techniques have manyinherent deficiencies which plaque fabrication processes and limitproduction of compositionally graded structures. Compositionally gradedstructures are those in which the proportion of a given element ormolecule varies with respect to the semiconductor matrix over thethickness of the structure. The production of such structures requiresthe capacity to vary the flux of a given molecular beam in apredetermined manner. In conventional MBE systems, flux rates of a givenmolecular (or atomic) species are determined by the temperatures, andassociated vapor pressures, of the corresponding source materials intheir effusion cells. In such conventional MBE systems, as described andillustrated in J. C. Anderson, et al., Materials Science 4 th Ed.,Chapman & Hall, N.Y., 1990, p. 464-6, selectively interposeableshuttering elements are positioned between beam sources and targetsubstrates which, upon actuation into or out of the path of themolecular flow, affect the initiation or termination of epitaxialgrowth.

The fabrication of compositionally graded structures, as describedabove, requires a more graduated variation in the beam flux than may beprovided by the conventional shuttering elements described in the priorart. While the obvious extension of the shuttering technique, whichcomprises partially inserting a shuttering element into the path of themolecular flow, would reduce the total flux of material, the resultantpartial beam would not be uniform. This is so even when the effects ofangular dispersion and spatial molecular averaging of the beam are takeninto consideration. Non-uniformity in molecular beams of the sortdescribed above cause lateral variation of molecular composition, notthe depth variation which is desired.

Several techniques have been disclosed in the art which relates tomethods and mechanisms for controllably varying beam fluxes. First isthe method of adjusting the temperature, and thereby the vapor pressure,of the individual effusion cell. The levels of control, uniformity, andreproducibility of the beams and epitaxial layers generated by usingthis technique are limited. Significant time lags in adjusting thetemperature of the source elements contribute to the uniformity problemsas well as to a dramatic slowing of the production process. Duringtemperature adjustment periods, the flow of source material must beinterrupted by interposing shutter elements. During these interruptedperiods of temperature adjustment, however, effusion from the source isnot terminated. As a result of this, and because the shutter elements donot make a physical seal, a deposition of leaked material continues atan unregulated rate.

The second of the techniques for controllably varying beam fluxesincludes the use of expensive needle valves, referred to, in combinationwith the effusion cells, as valved cracker cells. For example, U.S. Pat.No. 5,080,870 discloses an MBE system including a valve which controlsmolecular flux. In U.S. Pat. 5,080,870, and other valved cracker cellMBE systems, source material within an effusion cell is maintained at aconstant temperature, thereby sustaining a constant vapor pressurewithin the cell. Opening the needle valve permits the comparativelyhigher pressure vapor within the cell to escape at a specific flux intothe evacuated main MBE chamber. The resultant molecular beam issubsequently directed to a substrate. The extent to which the valve isopened determines the molecular flux of the beam.

Unlike conventional effusion cells, which produce beams of particleshaving an average energy directly proportional to the temperature of thesource material, the energy and speed of the beam produced by a valvedcracker cell is, in large part, determined by the difference in pressureacross the valve. Forced expansion of the heated vapor through a valvecauses it to cool. Valved cracker cells are, therefore, reasonablysuccessful for use with materials which have relatively low boilingpoints. Arsenic is one such element, however, the majority of importantmaterials (i.e. Ga, Al, In, Sb, Cd, Zn, Se, and Te) have higher boilingpoints. If these materials are used as sources within valved crackercells, expansion of the heated vapor through the needle valves tends tocause condensation of the material within the valve. Over a short periodof operation, the condensing vapor narrows the effective throat of thevalve, thereby altering its performance. The build up of condensedmaterial, in fact, may eventually clog the valve completely. Thesedifficulties, which reduce the ability to effectively regulate of theflux of the beam, render valved cracker cells unsuitable for use in manyMBE operations in the fabrication of advanced photonic and electronicdevices with compositionally graded regions.

It is, therefore, an object of the present invention to provide amechanism for use in combination with an effusion cell which affectsrapid and controlled variation of the flux within a molecular beam whichdoes not require adjustment of the temperature of the source material.

It is still another object of the present invention to provide amechanism for rapidly varying the flux from an effusion cell whichpermits the use of a wide variety of important source materials.

It is still another object of the present invention to provide amechanism for rapidly varying the flux of a molecular beam which doesnot use a valve and, therefore, does not become clogged by a condensingsource vapor.

It is still another object of the present invention to provide a rapidflux varying effusion cell assembly which permits faster and lessexpensive MBE fabrication of compositionally graded devices.

Other objects and advantages of the invention will be more fullyapparent from the ensuing disclosure and appended claims.

SUMMARY OF THE INVENTION

The foregoing objects are achieved by providing within an effusion cell,a means for controllably adjusting the angular distribution of amolecular field of vapor effusing from a source material within theeffusion cell. In one aspect of the present invention the means is atranslatable orifice mounted within an effusion cell; the position ofthe orifice being selectively and motively translatable within theeffusion cell for the purpose of adjusting the distance between theorifice and the source, thereby effecting a change in the flux of sourcevapor effusing from the cell.

In one such aspect, the traveling orifice of the present invention isdefined by a hollow frustoconical section having a open base (the narrowend of the section) and an open top (the wider end of the section). Thesection is slideably mounted within a crucible in which a sourcematerial is also disposed. When the source is heated within the crucibleat a position between the base of the crucible and the base of themotive frusto-conical section, and the assembly is mounted within asufficiently evacuated chamber, a molecular beam emanates from thesource, passes through the frusto-conical section, and is directedtherefrom to a suitable substrate to produce an epitaxial layer.

Given the conventional angular distribution of vapor effusing from acell, flux control is established by translating the orifice(translating the frustoconical section) to selected positions whichadjust the distance between the source material at the base of thecrucible and the orifice. An orifice which is positioned closer to theheated source allows a greater number of molecules to pass through. Anincrease in the distance between the orifice and the source decreasesthe number of molecules which may traverse the orifice and impinge uponthe substrate.

In another such aspect, the traveling orifice of the present inventioncomprises a nozzle-like open throat which is translatably mounted withinan effusion crucible, mounted therein to permit repositioning of thethroat with respect to a heated and effusing source disposed therein.

In a different aspect, the means for controllably varying the flux areachieved by a selectively widenable throat mounted in an effusioncrucible, which throat is fixably positioned with respect to a sourcedisposed therein. A selectively widenable throat according to such anaspect of the present invention may comprise a plurality of interleavingcurvilinear members forming a frustoconical section. The distal ends ofthe interleaving members, remote from the effusing source material arefixedly hinged to the inner side wall of the crucible. The ends of theinterleaving members which are proximally disposed to the sourcematerial form a variable area orifice; the area of which may be enlargedor reduced by appropriate rotation of the interleaving members abouttheir hinge points.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating one aspect of the presentinvention.

FIG. 2 is a cross-sectional view which illustrates another aspect of thepresent invention.

FIG. 3 is a cross-sectional view illustrating still another aspect ofthe present invention.

FIG. 4 is an axial view of a variable area orifice according to oneaspect of the present invention.

FIG. 5 is a cross-sectional view illustrating another embodiment of thepresent invention which combines the elements of the invention shown inFIGS. 1 and 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the present invention is described more fully hereinafter withreference to the accompanying drawings, in connection with whichparticular embodiments are illustrated, it is to be understood thatpersons skilled in the art may modify the invention herein describedfrom the specific forms and structures illustrated, while stillachieving the purpose, function and result of this invention.Accordingly, the descriptions which follow are to be understood asillustrative and exemplary of specific structures, aspects, and featureswithin the broad scope of the present invention and not to be construedas limiting of the broad scope of the present invention.

The present invention, in its various aspects and embodiments comprisesa means and method for varying the flux of a molecular beam emanatingfrom an effusion cell including an apparatus which adjusts the angulardistribution of a molecular field effusing from a source material withinan effusion cell. In one embodiment, the present invention features atranslatable orifice.

Referring now to FIG. 1, a rapid varying molecular flux effusion cell isillustrated in a cross-sectional view. The effusion cell assembly 10 issecurably mounted on mounting struts 12 within an ultra high vacuumchamber of an MBE apparatus (not shown). In a preferred embodiment, theeffusion cell assembly 10 is mounted on a flange element 14 which formsa removable portion of the wall of the vacuum chamber. Such anembodiment permits easy removal, substitution, and replacement of sourceelements.

The effusion cell assembly includes an outerbody housing 16, of ageneral cylindrical shape, having a enclosed base 18 and a solid sidewall 20. The outerbody housing 16 of one preferred embodiment isconstructed of tantalum. It is understood that other structuralmaterials which are stable at high temperatures and have suitably lowvapor pressures (required in order to avoid contamination of theepitaxial layers being grown within the chamber) may be used in thealternative.

The outerbody housing 16 defines a region 22 therein. Within region 22is positioned an inner crucible element 24. The crucible element 24,being frusto-conical in shape, has a maximum outer diameter at its openend which is less than the inner diameter of the outerbody housing. Atthe open end of the crucible element 24 is an annular lip 26 whichextends radially outward at a position adjacent to the side wall 20 ofthe outerbody housing 16 at its open end. The longitudinal axis of thecrucible element is generally coaxial with the outerbody housing 16. Ina preferred embodiment, the crucible element 24 is constructed ofpyrolytic boron nitride (PBN). The relevant requirement for the materialof which the crucible element 24 is manufactured is that it becompatible (low reactivity) with respect to the source materials whichare to be heated within the crucible.

A heating coil having windings 28 is positioned between the innersurface of the outerbody housing 16 and the outer surface of thecrucible element 24, along the length of the generally cylindrical innersurface of the housing side wall 20. The heating coils 24 are used toheat the crucible and a source material 30 disposed therein, thusraising the vapor pressure of the source within the crucible above thelow pressure of the vacuum chamber, therein causing the source toeffuse, generating a generally expanding molecular field of vapor.

A translatable hollow frusto-conical section 32, having a cone angle andwide end diameter equal to those of the crucible, is coaxiallypositioned within the crucible 24. The axial length of the section 32 isless than the length of the crucible; a region 34, therefore, existingbetween the inner end of the section 32 and the base of the crucibleelement 24 in which the source material 30 is heated. The narrow end ofthe section 32 forms an orifice 36 through which, when the sourcematerial is effusing, a portion of the expanding molecular field passesand escapes from the crucible, therein forming a beam.

The wide end 38 of frusto-conical section 32 is attached to an annularring support 40 which extends radially from the wide end of the section32 beyond the outer diameter of the outerbody housing 16. A pair ofdiametrically remote, longitudinally extending, bore holes 42 aredisposed through the cylindrical side wall 20 of the outerbody housing16. A pair of support rods 44a and 44b having a diameter smaller thanthe bore holes 42, are connected to the annular ring 40, and aretranslatably disposed within the bore holes 42 in such a manner that thesection 32 forming the orifice 36 may travel forward and backward alongthe axial line of the assembly 10. Such repositioning may beautomatically carried out within the chamber by motively coupling amotor (not shown) to at least one of the support rods 44a,44b. Such amotor could be controlled by an operator external to the high vacuumchamber.

In the illustrated embodiment one of the support rods 44a is extendedthrough a vacuum sealing port 46 and terminates at a handle 48 thereinpermitting manual repositioning of the traveling section 32 and thereinthe orifice 36. The length of the handle 48 from the wall 14 is suchthat it limits the translating distance which the section 32 may travelout from the housing 16. This effective mechanical stop serves as ablock to prevent the section 32 from becoming dislodged during extremerepositioning. It is understood that in specific embodiments whichinclude internally disposed motive elements, as described above, othermeans may be employed which serve an equivalent function to set thestroke length of the element 50 so that it does not exceed the depth ofthe outerbody housing 16.

With the novel assembly 10, the flux of a molecular beam emanating froman effusion cell containing source material 30, heated to a constanthigh temperature, can be controllably varied by adjusting the distancebetween the orifice 36 and the source. Given the conventional angulardistribution of vapor effusing from a source, positioning the orifice 36close to the source will allow a larger angle of that distribution, andtherein a greater number of gas molecules, to escape in the emanatingbeam. Positioning the orifice 36 farther away from the source will, inturn, reduce the number of effusing molecules emitted in the molecular

Referring now to FIG. 2, another aspect of the present invention isillustrated in a cross-sectional view. Similar elements shown in FIGS. 1and 2 are identified with similar numerals. This alternate embodiment ofthe invention, like the embodiment described above, is securably mountedon mounting struts 12 within an ultra high vacuum chamber of an MBEapparatus (not shown). The effusion cell assembly 10 may be mounted on aflange element 14 which forms a removable portion of the wall of thevacuum chamber.

As described above, this assembly includes an outerbody housing 16, of ageneral cylindrical shape, having a enclosed base 18 and a solid sidewall 20. Fixably positioned at the base of the outerbody housing 16 isan inner crucible element 24. A source material 30 may be positionedwithin the crucible element 24, which is frusto-conical in shape.

A heating coil having windings 28 is positioned between the innersurface at the base of the outerbody housing 16 and the outer surface ofthe crucible element 24. The heating coils 28 are used to heat thecrucible and the source material 30 disposed therein, thus raising thevapor pressure of the source causing it to effuse, generating amolecular field of vapor, from which a molecular beam may be isolated.

Mounted coaxially within the outerbody housing 16 is a translatablecylindrical element 50. The outer end 52 of the element 50 is connectedto an annular ring 54 and support rod 44a,44b assembly which is similarto that of the first embodiment. The total length of the element 50 isless than the depth of the housing 16, so as to position the inner end60 of the element 50 at the opening 21 of the crucible at the extremeinsertion of the element. The extreme inner extent of the stroke of theelement 50, corresponds to the position at which annular ring 54contacts the end portion of the outerbody housing.

With respect to the support rod 44a, of the present embodiment, whichextends beyond the vacuum chamber wall 14, as in the embodiment shown inFIG. 1, the length of the handle 48 from the wall 14 is such that itserves as an effective mechanical stop or block to prevent the element50 from becoming dislodged during extreme repositioning. Again, it isunderstood that in specific embodiments which include internallydisposed motive elements for translating element 50, other means may beemployed which serve an equivalent function to set the stroke length ofthe element 50 so that it does not exceed the depth of the outerbodyhousing 16.

The cylindrical element 50 has an outer diameter less than the innerdiameter of the outerbody housing, therein limiting the friction forcesopposing free adjustment of the longitudinal position of the elementwith respect to the crucible.

The interior surface of the traveling cylindrical element 50 includestwo geometrically distinguishable portions, an upper portion 62, and alower portion 64. The upper portion 62 defines a regular tubular shape,having a constant diameter. The lower portion 64, having a nozzle-like,frusto-conical section shape, narrows from an inner diameter equal tothat of the upper portion 62 to a reduced diameter, therein defining anorifice 36.

Repositioning the entire element 50 so that orifice 36 is close to theeffusing source provides a larger angle through which the vapor mayemanate. Moving the element 50, and therein the orifice 36, comparablyfarther from the source 30, as shown by the phantom lines in FIG. 2,which lines correspond to a translation of the element, reduces theangular distribution of the total effusing material which may pass. Theconstant area of the upper portion 62, through which the emanatingmolecular beam passes, ensures that the beam area is relativelyconstant, and independent of the distance from the orifice to the source(and therein independent of the flux in the beam).

Referring now to FIG. 3, a different aspect of the present invention,including a further alternative apparatus for altering the distributionof an effusing molecular field which comprises the molecular beam, isshown in a cross-sectional view. In such an embodiment, the means forcontrolling the distribution of the emanating molecular field comprisesan orifice having a selectively adjustable opening area. FIG. 3 shows anouterbody housing 16 which is secured to a wall 14 of a ultra highvacuum molecular beam epitaxy chamber by mounting struts 12. Theouterbody housing 16, includes a base 18, generally cylindrical sidewalls 20, and an open end 21, and contains a crucible 24 fixedly mountedto the inner surface of the base 18 of the housing. Heating coils 28 arepositioned between the inner surface of the outerbody housing 16 and theouter surface of the crucible 24; the coils 28 being positioned to heata source material 30 disposed within the crucible, therein producing amolecular field of vapor.

A cylindrical assembly of interleaved curvilinear members 66 is fixedlyhinged, by hinges 68, to the outerbody housing 16 at the inner surfaceof the side wall 20 at the open end 21. The interleaved curvilinearmembers 66 form a cylindrical sheath within the outerbody housing 16,extending into the housing and terminating above the crucible 24. Thearea defined by the circular cross-section of the interleaved members 66at the open end of the outerbody housing 16 remains constant because themembers 66 are radially fixed by the hinges 68. The ends 70 of theinterleaved members 66, which are closest to the source, and which areinterleaveably coupled, slide relative to one another, permitting thecircular cross-section defined by those ends 70 to be varied. Thecircular cross-section defined by the ends 70 of the members 66,therefore, defines an orifice 36 having a selectively widenablediameter.

Selective widening or narrowing of the orifice 36, which may be achievedby radially drawing out or compressing the interleaved members 66,alters the angular distribution of the field of effusing sourcematerials which may pass through the orifice to form a molecular beam.Means for radially drawing out or compressing the interleaved members 66is not shown. Suitable means include coupling the members 66 to anactuatable motor and a series of shafts which apply a radial force to asuitable number of members, causing the interleaved members 66 to drawtogether, therein narrowing the orifice 36. It is understood that avariety of alternative means maybe employed which serve the samefunction without distinguishing themselves from the broad scope of thepresent invention.

Referring now to FIG. 4, an orifice having interleaving curvilinearsections 66 according to one aspect of the present invention is shown inan axial view. Phantom lines 70 illustrate the direction of motion ofthe trailing edges 72 of the members 66. Leading edges 74 of the membersare understood to slide relative to the trailing edges 72 to which theyare adjacent, therein permitting the area of the orifice to be adjusted.

Referring now to FIG. 5, an aspect of the present invention whichincludes a variable area orifice which also travels with respect to thesource is illustrated in a cross-sectional view. The device shown hereinis a combination of the devices illustrated in FIGS. 1 and 3, thereinenabling an operator to vary the flux of a molecular beam by reducingthe angular distribution of the effusing source material which forms thebeam by either repositioning the orifice and/or widening or narrowingit.

In each aspect of the present invention, a source material is heated toproduce a molecular field having a density defined by the vaporpressure. By providing for the adjustment of an orifice size and/or theposition of the orifice within the molecular field, the presentinvention permits a selected angular distribution of the molecular fieldto be isolated, the isolated distribution therein forming a molecularbeam. The rapid and controllable variation in orifice size and/orposition therein adjusting the flux of the molecular beam.

While there has been described and illustrated apparati and methods forvarying the flux of a molecular beam produced by a molecular beamepitaxy cell, it will be apparent to those skilled in the art thatvariations and modifications are possible without deviating from thebroad spirit and principle of the present invention which shall belimited solely by the scope of the claims appended hereto.

What is claimed is:
 1. A molecular beam epitaxy source cell having arapidly and controllably varying flux, comprising:a crucible elementincluding a sealed base, side walls, and an opening, having a quantityof source material disposed therein; at least one heating element forraising the vapor pressure of the source material, thereby causingeffusion of the source material; a translatable conical section definingan orifice, interposed between the source material and said opening ofthe cell, slideably mounted within the crucible element; and means forselectively translating the conical section with respect to the effusingsource material, whereby translation of the traveling orifice alters theangular distribution of effusing material which emanates from the cell,thereby controllably varying the flux emanating from the cell.
 2. Themolecular beam epitaxy cell according to claim 2, wherein said crucibleelement comprises pyrolytic boron nitride.
 3. The molecular beam epitaxycell according to claim 1, wherein said conical section comprisespyrolytic boron nitride.
 4. A molecular beam epitaxy source cell havinga rapidly and controllably varying flux, comprising:a crucible elementincluding a sealed base, side walls, and an opening, having a quantityof source material disposed therein; at least one heating element forraising the vapor pressure of the source material, thereby causingeffusion of the source material; and a conical section forming anorifice, fixedly mounted within the crucible element, interposed betweenthe effusing source material and the opening, which orifice isselectively widenable, wherein widening of the orifice alters theangular distribution of effusing source material which emanates from thecell, therein varying the flux of the beam.
 5. The molecular beamepitaxy source cell of claim 4, wherein the selectively widenableconical section comprises: a plurality of interleaving curvilinearmembers having (a) first ends, remote from the effusing source material,which are fixedly hinged to an inner surface of said side walls of thecrucible, and (b) second ends, disposed nearer to the source material,which ends form a variable area orifice; the cross-sectional area ofsaid conical section being enlarged or reduced by appropriate rotationof the interleaving members about said hinges.
 6. A molecular beamepitaxy source cell having a rapidly and controllably varying flux,comprising:a crucible element including a sealed base, side walls, andan opening, having a quantity of source material disposed therein; atleast one heating element for raising the vapor pressure of the sourcematerial, thereby causing effusion of the source material; atranslatable conical section forming an orifice, interposed between thesource material and said opening of said cell, slideably mounted withinthe crucible element; and means for selectively translating the conicalsection with respect to the source material, said conical sectionfurther comprising a selective widenable orifice, whereby wideningand/or translating the orifice adjusts the angular distribution ofeffusing material which emanates from the cell, thereby vary the flux ofthe beam.
 7. A method of producing a molecular beam having a varyingflux, comprising;heating a source material within a crucible, having anopen top, therein causing the source material to effuse to produce amolecular field from which a beam may be isolated; selectively alteringthe angular distribution of said effusing molecular field whichcomprises the molecular beam, thereby varying the flux of the beam. 8.The method according to claim 7, wherein the step of selectivelyaltering the angular distribution of the molecular field comprising themolecular beam comprises, translating a fixed area orifice with respectto the effusing source material.
 9. The method according to claim 7,wherein the step of selectively altering the angular distribution of themolecular field comprising the molecular beam comprises, selectivelyadjusting the area of an orifice defined by an element mounted withinsaid crucible which element is fixedly mounted with respect to theeffusing source material.